1. Field
The present disclosure relates to methods for determining an amount of imbibition of hydraulic fracturing fluids into hydrocarbon-bearing rock formations. More specifically, the disclosure relates to laboratory methods for determining certain unconventional flow parameters to measure the imbibition over time of hydraulic fracturing fluids into a low-permeability hydrocarbon-bearing rock formation.
2. Description of the Related Art
Unconventional hydrocarbon shale gas resources present significant potential for meeting the world's ever increasing energy demand. Hydraulic fracturing of horizontal wells is a widely used technique for recovering natural gas from shale gas and other unconventional reservoirs, which are characterized in part by extremely low permeabilities. This hydrocarbon recovery method generally involves the use of large volumes of fracturing fluids along with proppants. Water-based fracturing fluids (that are currently most commonly used in hydraulic fracturing) comprise about 99% fresh or recycled water, complemented by the addition of chemicals like surfactants, friction reducers, biocides, clay stabilizers and scale inhibitors.
During hydraulic fracturing and the well shut-in stages in a shale gas reservoir, a significant amount of fracturing fluid will flow into surrounding shale formations through hydraulic fractures. A certain amount of the hydraulic fracturing fluids, oftentimes comprised mostly of water, will absorb into the shale rock, or other rock, in the hydrocarbon-bearing formation through a process referred to as “imbibition.” This is known, because while certain amounts of fracking fluids are recovered from formations, large amounts of fracking fluids are lost in the formation. Spontaneous imbibition is of interest, because it is a mechanism for liquid uptake by the formation during the relatively long well shut-in period after flow-back of some fracturing fluids to the surface. One definition of imbibition includes a process by which one fluid, for example water, displaces another immiscible fluid, such as an immiscible hydrocarbon.
The process of imbibition has many practical implications for shale gas recovery. Liquid imbibition can cause the loss of gas relative permeability and lead to chemically altered zones near fracture-matrix interfaces in hydrocarbon-bearing formations. Imbibition of hydraulic fluids, such as water, into shale rock may reduce hydrocarbon recovery from a well. Darcy's law describes generally the flow of fluids through porous media. However, in part because of the strong solid-liquid interactions in low-permeability media, such as, for example, shale rock, Darcy's law is not always adequate for describing liquid flow patterns in a shale formation that become unconventional. “Tight” rock formations are those that are low-permeability, and include shale rock, tight carbonate, and tight sandstone.
Therefore, there is a need for accurate and efficient methods for measuring unconventional imbibition of fluids into low-permeability hydrocarbon-bearing formations where unconventional flow can be present.